This invention relates to a carbon monoxide oxidizing catalyst particularly for removing carbon monoxide from an air containing it.
In general, carbon monoxide (referred to as CO hereinafter) evolves by uncomplete combustion of simple substances or compounds of carbon and becomes attached very firmly to hemoglobin in blood, forming CO-hemoglobin, which prevents remarkably the functions of blood to absorb and carry oxygen, resulting in developing acute poisonous symptoms such as headaches or dizziness, extremely causing death. Also it is said the long-term exposure to an air containing a high concentration of carbon monoxide induces chronic heart diseases.
CO oxidizing catalysts are used for converting harmful CO in an air into innocuous carbon dioxide (CO.sub.2). For example, they are packed in air cleaners, the absorption chamber of gas masks, or filters. At present, there is a need for the development of CO oxidizing catalyst not only capable of efficiently oxidizing CO and thereby cleaning air containing a high concentration of CO but also having a superior durability permitting a long-term use.
So far are known many CO oxidizing catalysts usable under natural conditions, i.e. ordinary ambient temperature, atmospheric pressure and humidity. They are roughly classified as follows:
(1) Metallic oxide catalysts PA1 (2) Precious metal catalysts PA1 (3) Redox catalysts.
(1) A typical known catalyst is the so-called hopcalite catalyst consisting essentially of manganese dioxide and copper oxide (disclosed in Japanese laid-open patent application Nos. 72988/1976 and 96399/1978). It however has a disadvantage of being affected by moisture which rapidly destroys the activity, and hence the combined use of it with a powerful dehydrating agent is necessary.
(2) Some catalysts containing a precious metal such as palladium or platinum have been disclosed in Japanese laid-open patent application Nos. 73344/1980, 149192/1978 and 137039/1980. Any of these has a disadvantage that though the effect of removing CO from an air is adequate when it is of a relatively low concentration (2000 ppm), their activity is poor for a relatively high CO concentration (1%) and decays in a short time.
(3) A redox pair catalyst is typical which is composed of palladium chloride and cupric chloride and known as the so-called Wacker catalyst. The catalyst which was developed for the purpose of synthesizing acetaldehyde from ethylene also has a high activity of oxidizing CO.
The mechanism of oxidation of CO in the presence of the Wacker catalyst is presented by the reaction equations (J. Air Pollution Control Assoc. 28: 253, 1978): EQU CO+PdCl.sub.2.2H.sub.2 O.fwdarw.CO.sub.2 +Pd.sup.. +2HCl+H.sub.2 O (1) EQU Pd.sup.. +(CuCl.sub.2).sub.2.2H.sub.2 O.fwdarw.PdCl.sub.2.2H.sub.2 O+Cu.sub.2 Cl.sub.2 ( 2) EQU Cu.sub.2 Cl.sub.2 +2HCl+H.sub.2 +1/2O.sub.2 .fwdarw.(CuCl.sub.2).sub.2.2H.sub.2 O (3)
If (1)+(2)+(3) EQU then CO+1/2O.sub.2 .fwdarw.CO.sub.2 ( 4)
A problem is encountered also in this catalyst in that when carried on a porous carrier and used over a long time as a CO oxidizing catalyst, its temperature is raised by the oxidation heat of CO, and HCl is produced as expressed by equation (1) and volatilized away from the system, thus the redox cycle becomes gradually unbalanced, resulting in a fall in the CO oxidation rate.